1. Field of the Invention
The present invention relates to an optical fixing unit including a luminous-element array, which is used as a light source to emit fixing rays toward a thermosensitive recording material and in which a large number of luminous elements are arranged in matrix. The present invention further relates to an illuminance correcting method for the optical fixing unit and relates to a thermal printer using the optical fixing unit.
2. Description of the Related Art
There are color thermal printers for obtaining a full-color print. This kind of the thermal printer uses a color thermosensitive recording paper in which at least three thermosensitive coloring layers coloring in different colors are stacked. Thermal sensitivity of the coloring layer is lower as a position thereof is lower. The first and second thermosensitive coloring layers, which are respectively the uppermost layer and the adjacent layer thereof, possess fixation properties caused by ultraviolet rays of specific wavelength ranges. In the color thermal printer, while the color thermosensitive recording paper is reciprocated in a feeding direction, a thermal head disposed in a scanning direction is pressed thereon to perform thermal recording for the respective coloring layers. After the thermal recording has been performed for each coloring layer, the ultraviolet rays are applied by using a fixing unit to fix the thermally-recorded coloring layer so that the upper coloring layer is prevented from coloring when the thermal recording is performed for the lower coloring layer.
As a light source of the fixing unit, is used a mercury fluorescent lamp having a tube shape. A section of the mercury fluorescent lamp is generally circle so that the ultraviolet rays are uniformly radiated around the mercury fluorescent lamp. Thus, a reflector is disposed near the mercury fluorescent lamp in order to reflect the ultraviolet rays, which are uselessly radiated, toward the color thermosensitive recording paper. The mercury fluorescent lamp to be used has a double length of a width of the thermal recording paper (relative to the scanning direction) in consideration of light-amount attenuation caused at both ends of the mercury fluorescent lamp. Thus, the mercury fluorescent lamp has a disadvantage regarding arrangement space so that it is difficult to downsize the printer.
When the fixing unit has a large size, a conveyance distance of the color thermosensitive recording paper becomes long and printing time increases. Since the mercury fluorescent lamp greatly depends on a temperature of a light amount, a control circuit is necessary for controlling the light amount in accordance with variations of the temperature. Further, since the light amount changes with the passage of time, periodic maintenance is required. As stated above, there are problems concerning the manufacture cost and the maintenance.
On account of the above, an optical fixing unit using a luminous-element array is proposed. The luminous-element array includes a large number of small luminous elements, which are arranged in a scanning direction and a feeding direction so as to form a matrix. The luminous element is a light emitting diode, for example.
However, the luminous-element array uses many luminous elements. Thus, there arises a problem in that illuminance of the respective luminous elements vary. Further, there arises another problem in that illuminance distribution becomes uneven in the scanning direction due to a mounting error of the luminous element.
In order to even out the illuminance distribution in the scanning direction, it is considered to repair all of the defect elements, which are not turned on, by exchanging and rewiring the element. However, the luminous element is small and wiring thereof is fine. Thus, there is a problem in that it takes a lot of labors for a repair operation.